Heroin addiction is a chronic relapsing disorder associated with significant socioeconomic and public health consequences. Heroin addicts experience severe dysphoria and aversive withdrawal symptoms when drug use is discontinued. These negative responses are crucial factors that motivate relapse in many individuals, even after extended periods of abstinence. Little is known about the neurobiological basis of these responses across the post-withdrawal period, or how this neurobiology contributes to an individual's likelihood of relapse. Existing evidence suggests that the kappa-opioid receptor system plays a key role in states of aversion, dysphoria and depression. The proposed experiments aim to examine the extent to which kappa-opioid receptors drive the negative affective and reward-related states that emerge during heroin withdrawal, persist despite abstinence, and motivate relapse. These experiments will also identify whether kappa-opioid receptors contribute to an individual's vulnerability to relapse. The following Aims of this proposal will use a clinically relevant model of heroin addiction, in which rats are chronically exposed to escalating doses of heroin, to address these questions. Aim 1 will measure the distribution and signaling function of kappa-opioid receptors across the post- withdrawal period. These findings will provide important details about the nature of the persistent neurobiological changes associated with heroin withdrawal. Aim 2 will identify if the degree of these kappa- opioid receptor changes corresponds to the magnitude of heroin preference and/or withdrawal-related symptoms expressed by individual rats across the post-withdrawal period. These findings will identify potential behavioral and/or neurobiological risk factors contributing to an individual's likelihood of relapse. Aim 3 will determine the extent to which these behavioral responses are mediated by kappa-opioid receptors within critical motivation and affective pathways of the brain. An innovative technique using small interfering RNAs will be used to reduce the expression of kappa-opioid receptors on neurons in the ventral tegmental area. As siRNAs have significant therapeutic potential, these findings will inform the development of clinical interventions for the long-term treatment of drug addiction and prevention of relapse. Together, these proposed studies will extend our understanding of how kappa-opioid receptors function in heroin-dependent individuals, and how these receptors contribute to the dysphoria and aversive states that characterize the post-withdrawal period. This research will also provide insight into potential kappa-mediated mechanisms of individual vulnerability to addictive drugs, which represents an important step in the development of personalized approaches to the long-term prevention of relapse. Ultimately, this research will broaden our understanding of how endogenous opioid-receptor systems may be modulated as an approach to addiction therapy.